Dams could signal death knell for Mekong giant catfish

Bangkok, Thailand – Damming the mainstream of the lower Mekong River would represent a significant new threat to the survival of the Mekong giant catfish, one of the world’s largest and rarest freshwater fish, according to a new study commissioned by WWF.

The study sheds new light on the status of this elusive species, including data on its numbers, distribution, threats and measures needed to prevent its disappearance. While the exact population size is unknown, there could be as few as a couple of hundred adult Mekong giant catfish fish left.

According to the study, the Xayaburi dam on the Mekong mainstem in northern Laos would prove an impassable barrier for the migratory giant catfish – which are capable of reaching up to three metres in length and weighing as much as 300kg – and risks sending the species to extinction.

“A fish the size of a Mekong giant catfish simply will not be able to swim across a large barrier like a dam to reach its spawning grounds upstream,” said the study’s author and associate research professor at the University of Nevada, Dr. Zeb Hogan.

“These river titans need large, uninterrupted stretches of water to migrate, and specific water quality and flow conditions to move through their lifecycles of spawning, eating and breeding.”

Species in steep decline

Numbers of Mekong giant catfish are already in steep decline due to overfishing, habitat destruction and dams along the Mekong’s tributaries. In the Mun River, the largest tributary to the Mekong, a dam already blocks the migrations of the Mekong giant catfish and has isolated the Mun River from the remainder of the Mekong river basin. The study claims that the controversial Xayaburi dam could disrupt and even block spawning, and increase mortality if the fish pass through dam turbines.

“It’s likely the Mekong giant catfish use the stretch of river of the Xayaburi dam as a migration corridor, with adult fish likely passing through this area on their migration from floodplain rearing areas to upstream spawning sites,” added Dr. Hogan. “It is also possible the giant catfish spawn in the area where the dam is now located.”

Environment and water ministers had agreed at the Mekong River Commission meeting in 2011 to delay a decision on building the Xayaburi dam pending further studies on its environmental impacts. This agreement was swept aside last November when Laos decided to forge ahead with construction.

Dam fish passages unproven

Criticism of the US$3.5-billion Xayaburi project has been growing with concerns centred on the serious gaps in data and failures to fully account for the impacts of the dam, particularly concerning fisheries and sediment flows.

Pöyry, the Finnish firm advising Laos on the dam construction, argues that “fish passages” can be built to enable fish to get past the dam’s turbines and swim up and down the river. But this claim has never been successfully put into practice.

“You can’t expect fish ladders to work without understanding your target species, their swimming capabilities, and the water current that will attract these fish toward the pass entrance,” said Dr. Eric Baran with the World Fish Centre. “Research is still needed to ensure mitigation efforts will work.”

Mekong giant catfish were once widely distributed through the Mekong river basin, possibly as far as Myanmar and south-western China, and were relatively abundant up until the early 1900s. Their numbers have since plummeted and the species is now limited to the Mekong and its tributaries in Cambodia, Laos and Thailand.

Catch figures also offer sobering evidence of the species decline, with numbers dropping from thousands of fish in the late 1880s, to dozens in the 1990s, and only a few in recent times. Despite laws being in place in Thailand, Laos and Cambodia to regulate fishing for Mekong giant catfish, with a ban on fishing the species in Thailand and Cambodia, the species is still fished illegally and caught accidentally in fisheries targeting other species.

“Catches should be monitored to ensure that Mekong giant catfish are not being illegally targeted by fishers,” added Dr. Hogan. “Incidental catch should also be monitored since it is one of the best and only sources of information about the distribution, life history and abundance of this river giant.”

Urgent efforts needed to save the species

The study identifies key measures to prevent the river giant’s disappearance, including urgent efforts to safeguarding migratory corridors and critical habitat, and increased international cooperation, such as basin-wide management planning, since the species occurs in an international river and crosses country borders to complete its life cycle.

“The Mekong giant catfish symbolizes the ecological integrity of the Mekong River because the species is so vulnerable to fishing pressure and changes in the river environment. Its status is an indicator of the health of the entire river, and its recovery is an important part of the sustainable management of the Mekong basin,” said Dr. Lifeng Li, Director of WWF’s Global Freshwater Programme.

“The Mekong giant catfish can be saved, but it will take a level of commitment from all lower Mekong countries, as well as international organizations and donors, that currently does not exist.”

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Mekong giant catfish captured at the dais, or floating bagnets, in the Tonle Sap River in Cambodia. Several Mekong giant catfish are captured each year in the Tonle Sap River, usually in October and November as the floodwater recedes, the Tonle Sap Lake empties, and fish like the Mekong giant catfish move from the flooded areas of the lake to the mainstream Mekong River.

Mekong giant catfish captured in the dai fishery in Cambodia’s lower Tonle Sap river on October 21, 2002. The dais, or bagnets, are large cone-shaped nets that catch fish as they move out of the Tonle Sap Lake and into the Tonle Sap River and mainstream Mekong. Dais are not selective in terms of the fish that they catch and so sometimes catch endangered species like Mekong giant catfish and giant carp as by-catch. These fish are often injured when they are caught and sometimes die before they can be released.

Mekong giant catfish caught in the Tonle Sap River dais in Cambodia. The Tonle Sap Lake, Tonle Sap River, and Mekong River between Vietnam and northern Thailand and Lao PDR represent the limit of distribution of Mekong giant catfish at this time. Efforts to reduce harvest should focus on the Tonle Sap Lake, the Tonle Sap River, and hot spots of capture along the mainstream Mekong.